1. Field
This invention relates generally to a thermal interface for drawing heat from an electronic device and, more particularly, to a planar heat pipe for drawing heat from an electronic device mounted on a printed circuit board (PCB), where the heat pipe includes a plate portion mounted to the device and a flexible portion coupled to the plate portion that compensates for a coefficient of thermal expansion (CTE) mismatch between the heat pipe and the device.
2. Discussion
Solid-state electronic devices generate heat as they operate, which if significant enough can have a damaging affect on the device either through loss of performance or actual damage to the device. As solid-state electronic devices get smaller and operate at higher power, the heat generated by the device becomes more of a concern.
Solid-state electronic devices often employ some type of thermal interface or heat sink that draws heat away from the device during its operation. Often times these heat sinks rely on the flow of air around the heat sink and the device to help reduce the temperature. However, for space-based electronic devices, where air does not exist, removing heat from electronic devices becomes more challenging. Heat sinks for space-based applications typically require the device to be coupled to a large thermally conductive structure that may be bolted or glued to the PCB, and then to the spacecraft structure itself.
Various types of electronic devices, such as large flip-chip, array-area parts, high-power application specific integrated circuits (ASICs), field programmable gate arrays (FPGA), etc., sometimes employ a fine-pitch ball grid array (BGA) or column grid array (CGA) interconnection possibly having several hundred or even a few thousand conductive balls or electrically conductive columns that provides the electrical connection between the device and the leads on the PCB, and also provides the mounting structure for the device to the PCB. In this type of circuit design, it is necessary to provide a heat sink on the opposite side of the device from the PCB so that the heat path for the device is through the top of the device to the thermally conductive structure.
Because there usually is a significant CTE mismatch between the thermally conductive structure being made of metal and the ceramic-based device the conductive structure and the device expand differently when heated, which causes a lateral stress on the BGA or CGA. Because space-based electronic circuits may undergo significant thermal cycling and are often subjected to significant vibrations, that stress on the BGA or CGA affects the life of the device, where eventually connections, such as solder joints, at the BGA or CGB will fail. Non-metallic elastomeric materials have some of the necessary heat removal properties and can be used as heat sinks, but typically rely on significant sustained compressive loads to obtain adequate thermal conduction. The affects of such large static loads are not well established, but are generally assumed to be undesirable. Moreover, the long-term effect of a vacuum environment on the performance of such elastomers is a concern.
A heat pipe is a well known heat-transfer device that has many applications for heat removal. A heat pipe typically includes a sealed pipe or tube made of a high thermally conductive material, such as copper. A chamber within the heat pipe is partially filled with a sintered wicking material and a working fluid, such as water, where air is removed from the remaining volume of the chamber. The working fluid is a liquid at low temperature, and when it is heated by the device, turns into a vapor that travels through the chamber from a hot end of the heat pipe to a cold end of the heat pipe where it condenses back into a liquid. The liquid is then returned to the hot end of the heat pipe through the wicking material under capillary action that occurs as a result of the water being vaporized at the hot end.